1. Technical Field
This invention relates generally rechargeable batteries, and more specifically to a fuel gauging circuit that employs a safety circuit to determine the remaining capacity of a rechargeable cell.
2. Background Art
Portable electronic devices, like cellular telephones, pagers and two-way radios for example, derive their portability from rechargeable batteries. Such batteries provide users freedom in that they no longer need to have a cord tethered to a wall outlet to use these devices.
While many people may think that a rechargeable battery is simply a cell and a plastic housing, nothing could be further from the truth. Rechargeable battery packs often include circuit boards, electronic circuitry, mechanical assemblies and electromechanical protection components. The circuits employed in rechargeable battery packs include charging circuits that start, ramp, taper and stop current, and measure temperature, just to name a few of the functions. Simply put, a battery pack is a complex system of components working in harmony to safely deliver power to a portable electronic device.
One of the most common consumer issues with rechargeable battery packs is determining how much “charge” is left in the battery. Many a foul word has been uttered by a person who grabs a battery, attaches it to a portable device, and walks out of the house, only to find that the battery is dead. Some people use multiple batteries with a particular electronic device, and switch back and forth between the batteries during the day. Nothing is more frustrating that not knowing which one is the “charged” battery and which one is “dead”.
One prior art solution to this problem is a Coulomb-counting fuel gauge circuit. These fuel gauging circuits typically employ a current sensing resistor to measure the current flowing in and out of a rechargeable cell in a battery pack. As such, they “count” the Coulombs of current flowing in or out of the cell, and add or subtract this count from a total capacity stored in memory.
The problem with these fuel gauging circuits is two fold: First, unless they are perfectly accurate, they accumulate error. For example, if instead of measuring 1 Coulomb of current flowing out of the cell, the fuel gauging circuit measures 0.99 Coulombs, the capacity will be off by 1%. When the same Coulomb flows back into the cell, it will also be measured with the same 1% error, which is now compounded. Hence, the capacity will now be 2% off. As this process continues, the capacity gets less and less accurate.
The second problem is the cost. Coulomb-counting fuel gauge circuits are quite expensive when compared to the overall cost of a rechargeable battery pack. For this reason, manufacturers are often reluctant to include the fuel gauging circuitry in many battery packs.
There is thus a need for a simple, inexpensive fuel gauging circuit that allows a user to determine the approximate remaining capacity of a battery pack.